The present invention relates to estimation of a communications channel and adjustment of a receiver based on such estimation, and more particularly to Doppler bandwidth estimation for a receiver path and adjustment of a receiver equalizer based on such estimation. Even more particularly, the present invention relates to Doppler bandwidth estimation of a cellular telephone signal path, and selection of a maximum likelihood sequence estimating receiver equalizer based on such estimation.
Communication channels in the cellular environment commonly impose a combination of distorting effects on transmitted signals. Rayleigh fading, where a signal's perceived power level rises and falls rapidly over a wide range, results from the combination (interference) of signals that have traversed paths differing in length by at least a significant fraction of a wavelength (i.e., about 30 cm. for cellular). This type of interference is known as multi-path interference. Differences in path transmission times that approach the time taken to transmit a symbol result in a second problem called delay spread.
Delay spread results in reception of multiple delayed replicas of a transmitted signal. Each Rayleigh faded replica has randomly distributed amplitude and phase, and the rate at which this complex quantity varies is constrained by the Doppler bandwidth associated with a vehicle's speed, which is related to the velocity of, e.g., a mobile unit relative to a base station. In a frequency nonselective environment, the sampled outputs of a receiver's matched filter provide uncorrelated estimates of the transmitted data. As such, in terms of discrete time samples, the channel has exhibited an impulse response proportional to a delta function. With delay spread, on the other hand, the discrete time channel impulse response is extended to introduce energy at a number of symbol times. The effect of the channel on the transmitted signal, in turn, may be viewed as the convolution of the transmitted information with the channel's impulse response. The channel, therefore, emulates a convolutional coding process (encoder).
This leads to the possibility of estimating the transmitted information through the use of methods analogous to typical decoding of convolutional codes, i.e., maximum likelihood sequence estimation techniques. Unlike the more widely applied forward error correction decoding environment, the details of the encoding process in a reverse error correction decoding environment, are not known a priori by the receiver. Issues related to the need to estimate the form of the encoding process are addressed by this invention.
For the North American digital cellular system, a number of documents define the standards of implemented components. With respect to this invention, the following are of interest: "Dual-Mode Mobile Station-Base Station Compatibility Standard" denoted here as IS-54, EIA/TIA Project Number 2398, Rev. A January, 1991 and "Recommended Minimum Performance Standards for 800 MHz Dual-Mode Mobile Stations", denoted here as IS-55, EIA/TIA Project Number 2216, April, 1991.
Doppler shift, where a frequency of a received signal is either increased or decreased relative to a frequency of the signal when it is transmitted, is due to relative motion between a transmitter and a receiver. The amount of Doppler shift varies as a function of the speed and direction (i.e.,velocity) of the relative motion. Because a channel's phase is related to the Doppler shift, the rate at which the channels phase varies is a function of the relative velocity. When the relative velocity has a low speed, the rate of change in the channel is slower than when the relative velocity has a high speed. Thus, because noise, carrier frequency mismatch, and other effects inhibit channel estimation, channel estimation can be more accurate at slow relative velocity than at high relative velocity. This is due to the ability to perform such estimation over a greater period of time by averaging the received signal. When the relative velocity is high, such averaging must be performed over a much shorter period of time due to the rapid changes that are occurring in the channel (because older channel estimate information is not relevant to the changed channel). Typically, however, receivers, such as cellular telephone receivers, which are designed for use when there is some relative velocity between the receiver and a transmitter, are designed to estimate the channels phase at high relative velocity. Thus, the advantages of enhanced noise and carrier frequency mismatch tolerance, which are gained by averaging over greater periods of time, are sacrificed so that channel estimation can be performed quickly at high relative velocity.
It is therefore desirable to provide an enhancement in communications channel estimation, wherein the advantages of channel estimation at slow speed are gained, while providing rapid, albeit less accurate, estimation at high speed.
Thus, what is needed is a communications system that provides for adaptable channel estimation based on relative velocity between the receiver and transmitter, i.e., based on Doppler bandwidth. Such communications system should be able to adapt the channel estimation as a function of the relative velocity.
The present invention advantageously addresses the above and other needs.